1. Field
This invention is generally in the field of paper making, specifically in the field of coagulating and separating organic solids from liquors produced during the process of digesting raw materials in manufacture of pulp and paper.
2. State of the Art
Wood has been a primary source of cellulose fibers for paper making. Before use, the wood must be reduced to the fibrous state. This operation is called pulping. At present, commercial pulping operations are of three principal types: mechanical, full chemical, and semichemical. The process with which the invention is concerned is full chemical or semichemical pulping.
Full chemical pulping employs chemical reagents to effect a separation of the cellulose fibers from the other wood components. Wood chips are cooked with suitable chemicals in aqueous solution, usually at elevated temperatures and pressures. The object is to dissolve the organic binders termed "lignins", comprising up to 26% of the wood, along with other saccharide type organic molecules and other extraneous compounds, leaving the cellulose intact and in fibrous form. Though there is some cellulose degradation, the objective can be realized to a commercially satisfactory degree through the use of a variety of chemical reagents. Pulp yields are usually about 50% of the wood weight.
The kraft or sulfate process and the semikraft process are commonly employed. Here, the active pulping ingredients are sodium hydroxide, sodium carbonate, and sodium sulfide comprising an obviously strongly alkaline solution. Standard in kraft pulping is a liquor-recovery cycle, in which the dissolved organic constituents in the spent pulping liquors are burned for steam generation, and the inorganic pulping chemicals are recovered and reused.
The traditional digestion liquor recovery cycle consists of the step of evaporating the liquor to a high concentration, to a so-called "black liquor" or "black kraft liquor" which is about 70% solids. Other processes, such as vacuum flashing, may then be performed to increase solids even more. This high-solids content kraft black liquor is usually fed into a furnace, where black liquor combustibles are burned for energy recovery. The salts therein are collected in molten form from the furnace for recycle into the pulping process.
This process is disadvantageous because it requires a high capital cost plant investment and considerable energy consumption to effect evaporation to 70% or higher solids. The furnaces themselves are hazardous, and, in addition, there is the hazard of handling molten salt pools.
It has long been known that acidification of kraft black liquor to a pH of about 2 to 3 causes precipitation of an acid lignin in slimy, gelatinous form. In such form, it is difficult to separate by centrifugation, settling, or decanting. David M. Whalen described a simple method for precipitating lignin from kraft black liquor in Vol. 58, No. 5, May 1975 TAPPI Journal, pages 110-112. Whalen described a process whereby kraft black liquor is added slowly and with stirring to a mixture of an organic liquid, such as chloroform, and enough mineral acid to bring the final pH to about 2. The process was successful on a laboratory scale, but the large amounts of organic liquid required made the process impractical on a commercial scale. A more efficient way of separating out the lignins is still needed.